moderate system cleanup & refactor (#20355)

* system refactor, move out 600 lines

* compilation, slice, backwardsindex, misc_num moved out of system
* some procs/types moved into arithmetics, basic_types
* system no longer depends on syncio
* some procs moved around to fit with their surroundings

* make exceptions an import, old ops to misc_num

* move instantiationInfo back

* move back nim version, fix windows echo

* include compilation

* better docs for imported modules, fix unsigned ops

also remove ze, ze64, toU8, toU16, toU32 with nimPreviewSlimSystem

* fix terminal

* workaround IC test & weird csize bug, changelog

* move NimMajor etc back to compilation, rebase for CI

* try ic fix

* form single `indices`, slim out TaintedString, try fix IC

* fix CI, update changelog, addQuitProc

* fix CI

* try fix CI

* actually fix CI finally hopefully

* Update lib/system/compilation.nim

Co-authored-by: ringabout <43030857+ringabout@users.noreply.github.com>

* update kochdocs

* hopefully fix csize uses for slimsystem

* fix tquit

Co-authored-by: ringabout <43030857+ringabout@users.noreply.github.com>

changelog.md

@@ -7,8 +7,34 @@
 - `addr` is now available for all addressable locations,
   `unsafeAddr` is now deprecated and an alias for `addr`.
 
-- `io`, `assertions`, `formatfloat`, and `` dollars.`$` `` for objects are about to move out of the `system` module. You may instead import `std/syncio`, `std/assertions`, `std/formatfloat` and `std/objectdollar`.
-  The `-d:nimPreviewSlimSystem` option makes these imports required.
+- Certain definitions from the default `system` module have been moved to
+  the following new modules:
+
+  - `std/syncio`
+  - `std/assertions`
+  - `std/formatfloat`
+  - `std/objectdollar`
+
+  In the future, these definitions will be removed from the `system` module,
+  and their respective modules will have to be imported to use them.
+  Currently, to make these imports required, the `-d:nimPreviewSlimSystem` option
+  may be used.
+
+- Enabling `-d:nimPreviewSlimSystem` also removes the following deprecated
+  symbols in the `system` module:
+  - Aliases with `Error` suffix to exception types that have a `Defect` suffix
+    (see [exceptions](https://nim-lang.org/docs/exceptions.html)):
+    `ArithmeticError`, `DivByZeroError`, `OverflowError`,
+    `AccessViolationError`, `AssertionError`, `OutOfMemError`, `IndexError`,
+    `FieldError`, `RangeError`, `StackOverflowError`, `ReraiseError`,
+    `ObjectAssignmentError`, `ObjectConversionError`, `FloatingPointError`,
+    `FloatOverflowError`, `FloatUnderflowError`, `FloatInexactError`,
+    `DeadThreadError`, `NilAccessError`
+  - `addQuitProc`, replaced by `exitprocs.addExitProc`
+  - Legacy unsigned conversion operations: `ze`, `ze64`, `toU8`, `toU16`, `toU32`
+  - `TaintedString`, formerly a distinct alias to `string`
+  - `PInt32`, `PInt64`, `PFloat32`, `PFloat64`, aliases to
+    `ptr int32`, `ptr int64`, `ptr float32`, `ptr float64`
 
 - The `gc:v2` option is removed.